This invention relates to circular polarization plates and liquid crystal displays provided with same.
In recent years, a liquid crystal display has been highly expected to expand its market of the displays for personal digital assistants (PDA) where its thin and light characteristics can be fully utilized. Since such personal digital assistants (PDA) are generally driven with batteries, it has been necessary to keep the electricity to be consumed as low as possible. Therefore, an attention has been made particularly on reflective type liquid crystal displays for the personal digital assistants which are free of or need not use all the time an electricity-consuming back light and can be reduced in electricity consumption, thickness, and weight.
There have been widely used reflective type liquid crystal displays with two polarizers for black-and-white displaying in which a liquid crystal cell is sandwiched between a pair of polarizers and a reflector is disposed the outside of one of the polarizer. Furthermore, a reflective type liquid crystal display with a single polarizer in which a liquid crystal layer is sandwiched between a polarizer and a reflector has been proposed and practically used because it is brighter in principle than those with two polarizers and is easy to be colored as reported in JAPAN DISPLAY, 192 (1989) by T. Sonehara et al. However, these reflective type liquid crystal displays have a drawback that the displayed images become invisible when they are used in a dark place because they usually display the images using light from the outside of the displays.
To solve this problem, there has been proposed a transflective liquid crystal display with a single polarizer in which a transflector having characteristics that part of an incident light is allowed to transmit is used instead of the reflector and a back light, is provided, as described in Japanese Patent Laid Open Publication No. 10-206846. Therefore, this display can be used as a reflective type, i.e., reflection mode using light from the outside when the back light is not turned on and as a transmission type, i.e., transmission mode using the back light under the dark situations.
The transflective liquid crystal display with a single polarizer needs to make an incident of substantially circularly polarized light to the liquid crystal cell through the transflector in the transmission mode. Therefore, it is necessary to dispose a circular polarizer comprising one or a plurality of polymeric stretched films typically polycarbonates and a polarizer between the transflector and the back light. However, such a circular polarizer having polymeric stretched films has a problem that it is low in contrast and narrow in the viewing angle in the transmission mode. Furthermore, it has a problem that it is difficult to make the circular polarizer thin due to the thickness of the polymeric oriented film, i.e., 60 to 100 xcexcm, resulting in the increase of total thickness of the liquid crystal display.
The object of the present invention is to provide a circular polarizer which has excellent circular polarization properties and can be designed to be thin and a liquid crystal display which is bright in the transmission mode, high in contrast, and less dependence on the viewing angle.
That is, the present invention relates to a circular polarizer comprising at least a polarizer and an optical anisotropic element wherein the optical anisotropic element contains a liquid crystal film in which a twisted nematic alignment structure is fixed.
The present invention also relates to the circular polarizer in which the product of the birefringence xcex94n at a wavelength of 550 nm and thickness d (nm), of the liquid crystal film is set to be within the range of 140 nm or greater and 400 nm or less and the twisted nematic angle thereof is set to be within the range of 30 degrees or larger and 85 degrees or smaller.
The present invention also relates to the circular polarizer in which the combination of the product of the birefringence xcex94n at a wavelength of 550 nm and thickness d (nm) of the liquid crystal film and the twisted angle is set to satisfy any of the following conditions:
(1) 155 nm or greater and 175 nm or less and 40 degrees or larger and 50 degrees or smaller;
(2) 176 nm or greater and 216 nm or less and 58 degrees or larger and 70 degrees or smaller; and
(3) 230 nm or greater and 270 nm or less and 70 degrees or larger and 80 degrees or smaller.
The present invention also relates to the circular polarizer whose liquid crystal film is one obtained by aligning a liquid crystalline material in a liquid crystalline state so as to assume a twisted nematic phase and cooling the material as it is so as to fix the twisted nematic orientation in a glassy state.
The present invention also relates to the circular polarizer whose liquid crystal film is one obtained by aligning a liquid crystalline material in a liquid crystalline state so as to assume a twisted nematic phase and then photo- or thermal-crosslinking the material so as to fix the twisted nematic orientation.
The present invention relates to the circular polarizer which has at least a polarizer and an optical anisotropic element laminated one after another and a thickness of 300 xcexcm or less.
Furthermore, the present invention relates to a liquid crystal display having the aforementioned circular polarizer.
The present invention also relates to a transflective liquid crystal display which comprises at least a liquid crystal cell formed by sandwiching a liquid crystal layer between a pair of transparent substrates each provided with an electrode, a polarizer disposed on the viewer""s side of the cell, at least one optical retardation compensator disposed between the polarizer and the cell, and a transflector disposed in the rear viewing from the observer, of the liquid crystal layer wherein the aforementioned circular polarizer is provided in the rear viewing from the observer, of the transflector.